Supercapacitors (Ultra-capacitors or Electro-chemical Capacitors):
Supercapacitors are being considered for electric vehicle (EV), renewable energy storage, and modern grid applications. The high volumetric capacitance density of a supercapacitor (10 to 100 times greater than those of electrolytic capacitors) derives from using porous electrodes to create a large surface area conducive to the formation of diffuse double layer charges. This electric double layer (EDL) is created naturally at the solid-electrolyte interface when voltage is imposed. This implies that the specific capacitance of a supercapacitor is directly proportional to the specific surface area of the electrode material, e.g. activated carbon. This surface area must be accessible by electrolyte and the resulting interfacial zones must be sufficiently large to accommodate the EDL charges.
This EDL mechanism is based on surface ion adsorption. The required ions are pre-existing in a liquid electrolyte and do not come from the opposite electrode. In other words, the required ions to be deposited on the surface of a negative electrode (anode) active material (e.g., activated carbon particle) do not come from the positive electrode (cathode) side, and the required ions to be deposited on the surface of a cathode active material do not come from the anode side. When the supercapacitor is re-charged, local ions are deposited onto their respective local electrodes (typically via local molecular or ionic polarization of charges). There is no exchange of ions between an anode active material and a cathode active material.
In some supercapacitors, the stored energy is further augmented by pseudo-capacitance effects due to some electrochemical reactions (e.g., redox). In such a pseudo-capacitor, the ions involved in a redox pair also pre-exist in the same electrode. Again, there is no exchange of ions between an anode active material and a cathode active material.
Since the formation of EDLs does not involve a chemical reaction or an exchange of ions between the two opposite electrodes, the charge or discharge process of an EDL supercapacitor can be very fast, typically in seconds, resulting in a very high power density (typically 5,000-10,000 W/Kg). Compared with batteries, supercapacitors offer a higher power density, require no maintenance, offer a much higher cycle-life, require a very simple charging circuit, experience no “memory effect,” and are generally much safer. Physical, rather than chemical, energy storage is the key reason for their safe operation and extraordinarily high cycle-life.
Despite the positive attributes of supercapacitors, there are several technological barriers to widespread implementation of supercapacitors for various industrial applications. For instance, supercapacitors possess very low energy densities when compared to batteries (e.g., 5-8 Wh/kg for commercial supercapacitors vs. 10-30 Wh/Kg for the lead acid battery and 50-100 Wh/kg for the NiMH battery). Lithium-ion batteries possess a much higher energy density, typically in the range of 100-180 Wh/kg, based on the cell weight.
Lithium-Ion Batteries:
Although possessing a much higher energy density, lithium-ion batteries deliver a very low power density (typically 100-500 W/Kg), requiring typically hours for re-charge. Conventional lithium-ion batteries also pose some safety concern.
The low power density or long re-charge time of a lithium ion battery is due to the mechanism of shuttling lithium ions between an anode and a cathode, which requires lithium ions to enter or intercalate into the bulk of anode active material particles during re-charge, and into the bulk of cathode active material particles during discharge. For instance, as illustrated in FIG. 1(A), in a most commonly used lithium-ion battery featuring graphite particles as an anode active material, lithium ions are required to diffuse into the inter-planar spaces of a graphite crystal at the anode during re-charge. Most of these lithium ions have to come all the way from the cathode side by diffusing out of the bulk of a cathode active particle, through the pores of a solid separator (pores being filled with a liquid electrolyte), and into the bulk of a graphite particle at the anode.
During discharge, lithium ions diffuse out of the anode active material (graphite particles), migrate through the liquid electrolyte phase, and then diffuse into the bulk of complex cathode crystals (e.g. lithium cobalt oxide, lithium iron phosphate, or other lithium insertion compound).
These intercalation or diffusion processes require a long time to accomplish because solid-state diffusion (or diffusion inside a solid) is difficult and slow. This is why, for instance, the current lithium-ion battery for plug-in hybrid vehicles requires 2-7 hours of recharge time, as opposed to just seconds for supercapacitors. The above discussion suggests that an energy storage device that is capable of storing as much energy as in a battery and yet can be fully recharged in one or two minutes like a supercapacitor would be considered a revolutionary advancement in energy storage technology.
More Recent Developments:
Instead of using an inorganic lithium insertion compound, such as LiCoO2 and LiFePO4, that requires lithium insertion into and extraction from the bulk of an inorganic particle (typically 100 nm-20 μm, but more typically 1-10 μm in diameter), several attempts have been made to use organic molecules or polymers as an electrode active material for the cathode (lithium metal alone as the anode). For instance, Le Gall, et al investigated Poly(2,5-dihydroxy-1,4-benzoquinone-3,6-methylene) as an organic polymer cathode [T. Le Gall, et al. J. Power Sources, 119 (2003) 316-320] and Chen, et al used LixC6O6 organic cathode, obtained from a renewable source, in a lithium ion battery [H. Chen, et al. “From biomass to a renewable LixC6O6 organic electrode for sustainable Li-ion batteries,” ChemSusChem, 1(2008) 348-355]. In addition, X. Y. Han, et al. studied carbonyl derivative polymers [“Aromatic carbonyl derivative polymers as high-performance Li-ion storage materials,” Adv. Material, 19, 1616-1621(2007)] and J. F. Xiang, et al. studied a coordination polymer as a cathode [“A novel coordination polymer as positive electrode material for lithium ion battery,” Crystal Growth & Design, 8, 280-282 (2008)].
Unfortunately, these organic materials exhibit very poor electronic conductivity and, hence, electrons could not be quickly collected or could not be collected at all. Although these organic molecules contain carbonyl groups (>C═O) that could readily react with lithium ions (forming a redox pair), this redox mechanism was overwhelmed by the poor electronic conductivity. As a result, the battery cells featuring these organic molecules exhibit poor power densities. Le Gall et al added a large proportion of conductive acetylene black (typically 40-60% by weight) to partially overcome the conductivity issue; but, acetylene black significantly dilutes the amount of the active material. Further, the best achievable specific capacity of 150 mAhlg is far less than the theoretical specific capacity of 705 mAh/g of Poly(2,5-dihydroxy-1,4-benzoquinone-3,6-methylene).
Recently, more electrically conducting carbon nano-tubes (CNTs) containing carbonyl groups were used by Lee, et al to replace the organic molecules for use as a cathode material [S. W. Lee, et al, “High Power Lithium Batteries from Functionalized Carbon Nanotubes,” Nature Nanotechnology, 5 (2010) 531-537]. The significantly higher electronic conductivity of CNTs does serve to overcome the poor conductivity problem of organic molecules. However, the CNT-based electrodes prepared by the layer-by-layer (LBL) approach still suffer from several technical and economical issues. Some of these issues are:                (1) CNTs are known to be extremely expensive due to the low yield, low production rate, and low purification rate commonly associated with the current CNT preparation processes. The high material costs have significantly hindered the widespread application of CNTs.        (2) CNTs tend to form a tangled mass resembling a hairball, which is difficult to work with (e.g., difficult to disperse in a liquid solvent or resin matrix).        (3) The so-called “layer-by-layer” approach (LBL) used by Lee, et al is a slow and expensive process that is not amenable to large-scale fabrication of battery electrodes, or mass production of electrodes with an adequate thickness (most of the batteries have an electrode thickness of 100-300 μm). The thickness of the LBL electrodes produced by Lee, et al (a noted MIT research group) was limited to 3 μm or less.        (4) One might wonder how the thickness of the LBL CNT electrodes would impact their performance. The data provided by Lee, et al (e.g. Fig. S-7 of the Supporting Material of Lee, et al) show that the power density dropped by one order of magnitude when the LBL CNT electrode thickness was increased from 0.3 μm to 3.0 μm. The performance is likely to drop even further if the electrode thickness is increased to that of a useful battery or supercapacitor electrode (e.g., 100-300 μm).        (5) Although the ultra-thin LBL CNT electrodes provide a high power density (since Li ions only have to travel an extremely short distance), there was no data to prove that CNT-based electrodes of practical thickness could even work due to the poor CNT dispersion and electrolyte inaccesability issues. Lee, et al showed that the CNT-based composite electrodes prepared without using the LBL approach did not exhibit particularly good performance.        (6) CNTs have very limited amount of suitable sites to accept a functional group without damaging the basal plane or graphene plane structure. A CNT has only one end that is readily functionalizable and this end is an extremely small proportion of the total CNT surface. By chemically functionalizing the exterior basal plane, one could dramatically compromise the electronic conductivity of a CNT.        
Most recently, our research groups have reported, in two patent applications, the development of two new classes of highly conducting cathode active materials having a functional group that is capable of rapidly and reversibly forming a redox reaction with lithium ions. These materials are nano graphene (both single-layer graphene and multi-layer graphene sheets, collectively referred to as nano graphene platelets, NGPs) and disordered carbon (including soft carbon and hard carbon). These two patent applications are: C. G. Liu, et al., “Lithium Super-battery with a Functionalized Nano Graphene Cathode,” U.S. patent application Ser. No. 12/806,679 (Aug. 19, 2010) and C. G. Liu, et al, “Lithium Super-battery with a Functionalized Disordered Carbon Cathode,” U.S. patent application Ser. No. 12/924,211 (Sep. 23, 2010).
These new types of cathode active materials (used in the so-called lithium super-battery) include a chemically functionalized nano graphene platelet (NGP) or a functionalized disordered carbon material (such as soft carbon and hard carbon) having certain specific functional groups capable of reversibly and rapidly forming a redox pair with a lithium ion during the charge and discharge cycles of a battery cell. In these two patent applications, the functionalized disordered carbon or NGP is used in the cathode (not the anode) of the lithium super-battery. In this cathode, lithium ions in the liquid electrolyte only have to migrate to the edges or surfaces of graphene sheets (in the case of functionalized NGP cathode), or the edges/surfaces of the aromatic ring structures (small graphene sheets) in a disordered carbon matrix. No solid-state diffusion is required at the cathode. The presence of a functionalized graphene or carbon enables reversible storage of lithium on the surfaces (including edges), not the bulk, of the cathode material. Such a cathode material provides one type of lithium-storing or lithium-capturing surface. (There will be another type of lithium-storing surface, based on simple lithium deposition, to be discussed at a later section).
In conventional lithium-ion batteries, lithium ions must diffuse into and out of the bulk of a cathode active material, such as lithium cobalt oxide (LiCoO2) and lithium iron phosphate (LiFePO4). In these conventional lithium-ion batteries, lithium ions must also diffuse in and out of the inter-planar spaces in a graphite crystal serving as an anode active material. The lithium insertion or extraction procedures at both the cathode and the anode are very slow. Due to these slow processes of lithium diffusion in and out of these intercalation compounds (commonly referred to as solid-state diffusion processes), the conventional lithium ion batteries do not exhibit a high power density and the batteries require a long re-charge time. None of these conventional devices rely on select functional groups (e.g. attached at the edge or basal plane surfaces of a graphene sheet) that readily and reversibly form a redox reaction with a lithium ion from a lithium-containing electrolyte.
In contrast, the super-battery as reported in our two earlier patent applications (U.S. application Ser. No. 12/806,679 and Ser. No. 12/924,211), relies on the operation of a fast and reversible reaction between a functional group (attached or bonded to a graphene structure at the cathode) and a lithium ion in the electrolyte. Lithium ions coming from the anode side through a separator only have to diffuse in the liquid electrolyte residing in the cathode to reach a surface/edge of a graphene plane. These lithium ions do not need to diffuse into or out of the volume of a solid particle. Since no diffusion-limited intercalation is involved at the cathode, this process is fast and can occur in seconds. Hence, this is a totally new class of hybrid supercapacitor-battery that exhibits unparalleled and unprecedented combined performance of an exceptional power density, high energy density, long and stable cycle life, and wide operating temperature range. This device has the best of both battery and supercapacitor worlds.
In the lithium super-batteries described in these two patent applications, the anode comprises either particles of a lithium titanate-type anode active material (still requiring solid state diffusion), schematically illustrated in FIG. 1(B), or a lithium foil alone (without a nano-structured material to support or capture lithium ions/atoms), illustrated in FIG. 1(C). In the latter case, lithium has to deposit onto the front surface of an anode current collector alone (e.g. copper foil) when the battery is re-charged. Since the specific surface area of a current collector is very low (typically <<1 m2/gram), the over-all lithium re-deposition rate is relatively low (this issue is being overcome in the instant invention).
Herein reported is another superior energy storage device that also operates on lithium ion exchange between the cathode and the anode. However, in this new device, both the cathode and the anode (not just the cathode) have a lithium-capturing or lithium-storing surface (typically both being nano-structured with many lithium-storing surfaces) and both electrodes (not just the cathode) obviate the need to engage in solid-state diffusion. This is illustrated in FIG. 1(D) and FIG. 2. Both the anode and the cathode have large amounts of surface areas to allow lithium ions to deposit thereon simultaneously, enabling dramatically higher charge and discharge rates and higher power densities. The uniform dispersion of these surfaces of a nano-structured material (e.g. graphene, CNT, disordered carbon, nano-wire, and nano-fiber) in an electrode also provides a more uniform electric field in the electrode in which lithium can more uniformly deposit without forming a dendrite. Such a nano-structure eliminates the potential formation of dendrites, which was the most serious problem in conventional lithium metal batteries (commonly used in 1980s and early 1990s before being replaced by lithium-ion batteries). Such a device is herein referred to as a surface-controlled, lithium ion-exchanging battery.